A conventional optical module of this type has been disclosed in Japanese patent No. 2719804, for example. This conventional optical module comprises, as shown in FIG. 1, a planar microlens array 60 consisting of a planar transparent substrate having microlenses 61 formed in a surface thereof. Fitting recesses 65 are formed in a surface opposite to the lens-formed surface of the array 60, each of recesses 65 being aligned with the center of a corresponding microlens 61. An optical element to be optically coupled to the microlens 61 is an optical fiber 63, for example. The end core portion of an optical fiber is processed by a selective etching to form a micro fitting convex portion 66. According to the conventional optical module described above, an alignment can easily be conducted by inserting the convex portion 66 of an optical fiber into the fitting recess 65 to fix it thereto, instead of an active alignment (i.e., light is guided into an optical fiber and the position of the optical fiber is regulated so as to maximize light coupled to a microlens).
The conventional optical module described above causes the following problems in such a case that the optical module is used to combine especially with a planar optical element. That is, when the planar optical element is a planar transmission optical element module such as a liquid crystal switch, a microlens optical system must be constructed by a collimate optical system having an infinite conjugate ratio. FIG. 1 shows a microlens optical system constructed described above in which collimated light 100 exits from a microlens. On the other hand, in an optical module for connecting a light-emitting element such as a laser array to an optical fiber, a microlens optical system thereof is required to be constructed by a reducing image optical system having a finite conjugate rate. It should be noted that the conjugate ratio means the ratio of an object distance to an image distance.
In the case of an optical module coupled to optical fibers, a microlens optical system is required to be constructed by a unity magnification image optical system, while in the case of an optical module coupled to a planar optical element such as a photo-detector array having a light-receiving area larger than a mode field diameter of an optical fiber, a microlens optical system is required to be constructed by a magnification image optical system.
In order to satisfy these requirements, planar microlens arrays each having a different focal length of microlens are prepared separately so that an optimal conjugate ratio may be obtained for respective application in the conventional optical module, or the thickness of a substrate of planar microlens array is regulated to obtain an optimal conjugate ratio. As a result, the number of kinds of planar microlens arrays will be increased. Also, when the optical characteristic of an optical module is regulated in an experimental environment for example, it is often required to remake a planar microlens array. In such a case, an efficient development and early implementation of optical modules will be disturbed.
As an optical module in which the problems described above have been resolved, the applicant has already proposed an optical module comprising a planar transparent substrate for adjusting a conjugate ratio provided between a planar microlens array and a guide substrate for optical fibers.
Such a proposed optical module is shown in FIG. 2. The optical module comprises a planar microlens array 1, a transparent substrate 2 for adjusting a conjugate ratio of the optical module, a guide substrate 3 for optical fibers, and a plurality of optical fibers 4. The planar microlens array 1 consists of a planar transparent substrate, in one surface thereof a plurality of circular microlenses 11 are formed and arrayed. The transparent substrate 2 includes a plurality of micro fitting recesses 21 formed and arrayed in one surface thereof. The guide plate 3 includes a plurality of tapered micro guide holes 31 opened therethrough. The end core portion of each optical fiber 4 is exposed convexly to form a micro fitting convex portion 41. In the figure, reference numeral 5 designates adhesive, an index of refraction thereof being matched to that of the transparent substrate and guide plate.
In the conventional optical module shown in FIG. 1 and the optical module shown in FIG. 2, while a plurality of optical fibers may be aligned to respective microlenses, it is difficult to position all of the optical fibers perpendicularly to the planar microlens array.
If an optical fiber may not be positioned perpendicularly to the planar microlens array, the problem is caused such that a coupling efficiency is degraded. Especially, the effect of degradation of a coupling efficiency is remarkable in the case that the microlens optical system is constructed by a unity magnification optical system.
There are also some cases that all of optical fibers are required to be positioned to a planar microlens array with a predetermined angle inclined to the microlens array.